(1) Field of the Invention
This disclosure generally relates to cooled structures and more specifically to structures with adaptive cooling for use in hot environments such as found in gas turbine engines.
(2) Description of the Related Art
Gas turbine engines are used for powering military and commercial aircraft, ships and electrical generators. Turbine engines operate according to a continuous Brayton cycle where a compressor pressurizes incoming air, fuel is added and the mixture is ignited in a combustor to produce a flowing stream of hot gas. The hot gas is referred to as gas path air or primary air. The gas path air is compressed, used for combustion, and then expands through a turbine before exiting the engine as thrust. The turbine extracts energy from the gas path air and, in turn, powers the compressor via a common shaft. Some military applications may introduce fuel in an augmentor downstream of the turbine, where it is also ignited to increase thrust. Many gas turbine engine architectures are known in the art and this primer is provided merely as an overview.
A portion of the pressurized gas path air is bled from the compressor and bypasses the combustion process altogether. This air is referred to as cooling air or secondary air and is used to cool components in the engine that are in direct contact with the hot gas path air. In most instances, the temperature of the gas path air exceeds the melting temperatures of the combustor, turbine and augmentor components' base material, so cooling is indispensable.
Designers determine the volume of cooling air required to cool these components using computer models, rig simulations and engine tests. Unfortunately, the gas path air temperature tend to vary spanwisely and circumferentially around the engine at constant axial locations. Also, local hot spots can develop in the gas path air due to shifts in aerodynamics, hardware deterioration, cooling passage clogging and other such causes. It's often difficult for designers to predict where these local hot spots will occur, so surplus cooling air volume is provided to ensure adequate cooling margin exists everywhere, to mitigate the effects of a localized hot spot should one occur. An example of a combustor heat shield with adequate cooling margin is disclosed in U.S. Pat. No. 7,140,185 to United Technologies Corporation.
Brayton cycle efficiency generally improves with an increase in gas path air temperature and a decrease in cooling air volume. By providing excess cooling air where it is not actually required, cycle efficiency is reduced. What is needed is a structure with adaptive cooling so that only the optimal volume of cooling air is used and cycle efficiency is improved.